1. Field of the Invention
This invention relates generally to corona charging, and more particularly to an improved corona charger which is adaptable for a plurality of uses without physical adjustment to the charger. The charger is particularly suitable for electrostatographic machines.
2. Description of the Prior Art
Referring to FIG. 1, conventional corona charger designs for electrostatographic applications generally utilize a thin wire 10 surrounded by a grounded metal shell 12. Corona wire 10 is typically driven at a D.C. potential of say -5.4kV, which results in a characteristic plate current-to-potential curve such as shown in FIG. 2.
Although the charger design of FIG. 1 is adequate for transfer purposes, its performance is not optimum since it is voltage sensitive and does not act as a constant-current device. Accordingly, a charger set up to deliver the correct amount of charge for a receiver of one conductivity would not deliver the correct amount of charge if the receiver's conductivity changed because the charger is voltage sensitive.
Another problem with conventional chargers is that, due to the cutoff potential of the charger, they are not able to deliver sufficient charge under dry conditions. As the receiver potential approaches this cut-off, the wire potential, corona output is suppressed and current output to the receiver goes to zero. This could be overcome just by increasing the wire potential, but current output would then get excessively high, and high potentials result in overcharging in more humid conditions.
The problems mentioned above can be minimized by a focused corona charger such as the transfer station charger disclosed in commonly assigned, co-pending U.S. Pat. Application Ser. No. 104,469 filed Oct. 5, 1987 in the name of G. R. Walgrove, now Pat. No. 4,775,915 issued Oct. 4, 1988 which charger operates in a mode that better approximates constant-current operation and which reduces the transfer time. That focussed corona charger has a periodically energized corona wire and a non-conductive shell about the wire open towards a receiver surface. A conductive electrode is situated on the side of the wire opposed to the receiver. A voltage is applied to the wire and, with a time lag, to the electrode so that a useful amount of charge is generated by the wire before being accelerated to the receiver by the electrode when the voltage on the electrode approximates the voltage on the wire. A typical focussed charger characteristic plate current-to-potential curve in accordance with the charger disclosed in aforesaid U.S. Pat. Application Ser. No. 104,469 is shown in Figure 3. The characteristic curve shows a high cutoff (the point at which the curve intersects the x-axis) and a low slope. For the transfer operation, this is ideal, as it provide constant-current operation and minimizes sensitivity to receiver characteristics.
However, the transfer operation is not the only electrostatographic machine application for a corona charger. Chargers are also used for photoconductor charging, sheet detacking, photoconductor conditioning, etc. Each operation requires different operating characteristics from the charging device. That is, a different current-to-potential curve. It would be advantageous if the focussed charger could be used for all charging functions without physical adjustment to the charger.
Conventional corona charging techniques do not, however, allow programmability. The slope and cutoff of the chargers' characteristic plate current-to-potential curves can not be adjusted independently. Any attempt to adjust the cutoff by, for example, increasing the wire potential, results in a corresponding increase in the slope of the curve. To overcome this, typically a compromise is made where acceptable but not optimum charger performance is obtained. Often, physical adjustment of the charger with respect to the surface being charged is required to achieve even adequate performance. This increases the system cost and an adjustable charger mount is required.